Basics: The Sun
Main image: NASA SDO 4 April 2018
The Sun is a very large and hot object (otherwise known as a star) that is at the centre of the solar system. It is mostly hydrogen (92.1%) and the rest is helium. The temperature of the sun’s surface is about 10,340 degrees F (which is about 5,726 degrees Celsius). From earth it is about 93 million miles (150 million km).
The sun has a massive amount of gravity because it is so large. All objects with mass have gravity, but the larger objects have proportionally more. All celestial items in this solar system are either directly or indirectly orbiting around the sun. The moons are indirectly because they are orbiting their planet.
If you were really huge and had a massive bit of bread, hundreds of thousands of km wide, you could make toast with the sun. In fact, if you were in space you could probably toast bread with the sun yourself but at the same time you would toast yourself; it is unwise and somewhat infeasible.
It is the most important source of energy for earth. Examples of other sources of energy include things like wind, geothermal and water flow, etc. As can be noted though, vehicles tend to use the fossil fuels (which have limited supply).
The picture below shows Venus in transit in front of the Sun (Source: NASA/SDO, AIA, Published: June 5, 2012). To view live events like this, important steps have to be taken to avoid blinding yourself.
Solar flares and coronal mass ejections (CME) are 2 types of events that occur on the Sun. They are not the same. The picture below shows a CME (Source: Solar Dynamics Observatory, NASA, Published: April 3, 2018).
The following video from NASA provides information as to the difference between the 2 events: